With increasing technology developments and demands for mobile devices, demands for secondary batteries as an energy source have rapidly increased, and among such secondary batteries, lithium secondary batteries having high energy density and working potential, a long lifespan, and a low self-discharge rate have been commercialized and widely used.
Further, with growing concerns about environmental issues, many studies have been conducted on electric vehicles and hybrid electric vehicles which may be employed in place of vehicles using fossil fuels, such as gasoline vehicles, diesel vehicles, etc. which are one of major causes of air pollution. Although nickel hydrogen metal secondary batteries have been mainly used as a power source of the electric vehicles and hybrid electric vehicles, use of lithium secondary batteries having high energy density and discharge voltage has been actively studied, and some of them are now commercially available.
As a positive electrode material for the lithium secondary battery, LiCoO2, ternary materials (NMC/NCA), LiMnO4, LiFePO4, etc. are used.
Meanwhile, a technology for using a battery system in a high voltage band has been recently developed for high capacity of secondary batteries. Generally, secondary batteries have a charge/discharge range of 3.0 V to 4.2 V, but it has been studied that a higher charging voltage (4.2 V to 4.5 V) than the charging voltage is applied to obtain a higher energy capacity.
In particular, LiCoO2 has excellent physical properties such as high rolling density, etc., and excellent electrochemical properties such as high cycling property, and therefore, it has been frequently used until now. However, since LiCoO2 has a charge/discharge current capacity as low as about 150 mAh/g, it is necessary to apply a higher charging voltage (4.2 V to 4.5 V) in order to achieve high capacity.
However, when the charging voltage is set at 4.3 V or higher, there are problems in that a crystal structure of the active material becomes unstable due to increased oxidation power, deterioration of negative and positive electrodes becomes worse, as the charge/discharge cycle progresses, and change of Co3+ ions of lithium-cobalt oxide into Co4+ ions which are highly reactive to an electrolyte is increased to increase the decomposition reaction of the electrolyte, leading to rapid deterioration of surface stability of the active material and lifetime property of the battery and ignition by reactions with the electrolyte.
Accordingly, it is necessary to develop a lithium cobalt oxide-based positive electrode active material capable of ensuring surface stability without deterioration of performances at high voltages.